1. Forecasting Aircraft Icing Jonathan Leffler Chicago CWSU

2. Agenda  Definitions (review)  Considerations  Synoptic Conditions  Methodology  Mitigation Procedures  Icing Products & Reporting  Final Thoughts

3. Definitions  Types Rime: Whole droplet rapidly freezes, traps air, forms brittle, opaque, milky-colored ice. Most common form. Clear: Small part of drop freezes, rest smears along surface, less opaque, denser than rime. Greatest threat. Mixed: Combination of rime + clear. Can appear as layers of opaque and clear. Accumulates like clear, more difficult to remove than rime. Clear0 C to -10 C Mixed-10 C to -15 C Rime-15 C to - 40* C Common Temperature Ranges * Icing typically not found < -20 C to -25 C due to the minimal availability of supercooled liquid

4. Definitions  Rates of Accumulation Trace: Typically not hazardous to flight. De-icing, anti- icing and altitude change normally not required < 1 hr. Light: Occasional use of de-icing or anti-icing is required to remove or prevent accumulation. Could become problematic if duration > 1 hr. Moderate: Short encounter can be hazardous. Use of de-icing or anti-icing equipment is necessary and heading/altitude change normally required. Severe: De-icing or anti-icing equipment cannot control or remove hazard. Immediate heading/altitude change required.

5. Considerations  Seasonality: Fall through Spring, Summer TS  Supercooled large drops (solid ↔ liquid, e.g. FZRA)  Liquid water content, not ice content (hard to measure)  Temperature advection (warm/cold), 0 C & -20/-25 C  Droplet Size (rain/drizzle/cloud)  Depth & duration of moisture (horizontally/vertically)  PIREPS! What has already been reported?

6. Considerations Frequency of icing type occurrence

7. Synoptic Conditions

8. Cold Front  Usually not widespread  Faster moving  Clear more than Rime  Heavier ice w/Cu vs St Warm Front  More widespread clouds  Longer residency  Rime more than Clear  Heaviest in FZRA

9. Synoptic Conditions

10. Freezing Precip = Highest Threat  Snowflake to raindrop phase transition  Generates supercooled large drops (SLDs)  Greater moisture availability  Clear ice freezing process (dense, hard to remove)  Rapid accumulation → Severe

11. Synoptic Conditions 40-45 hr/yr 30-35 hr/yr

12. Methodology  Satellite: Visible & Infrared Surface moisture contamination (snow cover) Cloud type (stratus vs cumulus)  Observations Surface precipitation type (DZ, RA, SN) Heavy moisture in dendritic “snow” zone tends to scrub supercooled droplets (more freezing process) Cloud base & height  Radar + Sounding: Bright Band Height  AWIPS 4-Panel Interrogation (NWS) - Initial Areas  BUFKIT Soundings (Work/Home) - Refined Areas

13. Methodology ~ 050-070 SLD Threat MOD or greater potential 0 C

14. SFC-020 020-050 050-100 100-140

15. SFC-020 ? ?

16. 020-050 ? Lake Effect ?

17. 050-100

18. 100-140

19. RUC 18Z - KORD T + 4 (22Z) *Forecast* 075 015 Sampling Locations Snow Zone

20. RUC 18Z - KORD T + 9 (03Z) *Forecast* 015 075 Snow Zone

21. RUC 22Z - KORD T + 0 *Analysis* 055 035 Snow Zone

22. ISOL LGT ICGICIP AOB 070. The Forecast

23. ISOL LGT ICGICIP AOB 070. The Results What does MOD ICG look like?

24. RUC 18Z - KORD T + 4 (22Z) FWA 01 Feb/21Z 130 010 180 MOD LGT Snow Zone

25. CMI 01 Feb/21Z 130 005 180 MOD LGT SLD Threat Snow Zone

26. Mitigation Procedures  Ascend into colder layer (< -20 C), typically safest based on accumulation potential  Descend into warmer layer (> 0 C), but caution...  Lateral into FEW/SCT deck vs BKN/OVC deck Cold soaked skin = delayed warming and susceptible to icing with increased moisture

27. So where do I go now ?

28. Icing Products & Reporting http://aviationweather.gov/adds

29. Icing Products & Reporting

30. Graphical AIRMETS http://aviationweather.gov/products/gairmet

31. Icing Products & Reporting AIRMETS/SIGMETS

32. Icing Products & Reporting PIREPS Java Tool

33. Icing Products & Reporting http://aviationweather.gov/adds

34. Icing Products & Reporting Current Icing Potential (CIP) Forecast Icing Potential (FIP) CIP / FIP = Help pages on how icing analyses and forecasts are done

35. Icing Products & Reporting Experimental Icing Products

36. Icing Products & Reporting Experimental Icing Products

37. Icing Products & Reporting Experimental Icing Products

38. Final Thoughts  Icing can be a deadly aviation hazard if not planned for in advance.  Icing is subjective, however there are objective ways to forecast potential. We need your PIREPS!  Lots of web-based tools to help you flight plan  Graphics and literature courtesy AFWA TN-98/002  Special thanks to Robb Kaczmarek (Chicago CWSU) and Paul Merzlock (NWS Romeoville) for technical advice and support

39. Questions ?